The present invention relates to optical micro-electro-mechanical devices (xe2x80x9cMEMs devicesxe2x80x9d) and, in particular, to MEMs devices constructed using solder bond assembly. The inventive MEMs devices are particularly useful as movable mirrors in optical communication systems.
Optical MEMs devices are of considerable importance in optical communication systems. In one important application, a two-dimensional array of MEMs devices is used to provide an optical cross connect between input optical fibers and output optical fibers. Each MEMs device in the array is a movable mirror disposed to receive the optical signal from an input fiber. The mirror can be electrically moved to reflect the received optical input to a desired output fiber.
A typical MEMs mirror comprises a metal-coated silicon mirror movably coupled to a surrounding silicon frame via a gimbal. Two torsional members on opposite sides of the mirror connect the mirror to the gimbal and define the mirror""s axis of rotation. The gimbal, in turn, is coupled to the surrounding silicon frame via two torsional members defining a second axis of rotation orthagonal to that of the mirror. A light beam can therefore be steered in any direction.
Electrodes are disposed in a cavity underlying the mirror and the gimbal. Voltages applied between the mirror and an underlying electrode and between the gimbal and an electrode control the orientation of the mirror. Alternatively, in slightly modified arrangements, an electrical signal can control the position of the mirror magnetically or piezoelectrically.
FIGS. 1(a) and 1(b) illustrate conventional optical MEMs devices and their application. FIG. 1(a) shows a typical prior art optical MEMs mirror structure. The device comprises a mirror 10 coupled to a gimbal 11 on a polysilicon frame 12. The components are fabricated on a substrate (not shown) by micromachining processes such as multilayer deposition and selective etching. After etching, mirror assembly (10, 11, 12) is raised above the substrate by upward bending lift arms 13 during a release process. The mirror 10 in this example is double-gimbal cantilevered and attached onto the frame structure 12 by springs 14. The mirrors can be tilted to any desired orientation for optical signal routing via electrostatic or other actuation with electrical voltage or current supplied from outside. The light-reflecting surface of mirror 10 comprises a metal coated polysilicon membrane which is typically of circular shape. The metal layers are deposited by known thin film deposition methods such as evaporation, sputtering, electrochemical deposition, or chemical vapor deposition.
FIG. 1(b) schematically illustrates an important application of optical MEMs mirrors as controllable mirror arrays for optical signal routing. The cross connect system shown here comprises optical input fibers 120, optical output fibers 121 and an array of MEMs mirrors 122 on a substrate 123. The optical signals from the input fibers 120 are incident on aligned mirrors 122. The mirrors 122, with the aid of a fixed auxilliary mirror 124 and appropriate imaging lenses 125, are electrically controlled to reflect the incident optical signals to respective output fibers 121. In alternative schemes, the input fibers and the output fibers are in separate arrays, and a pair of MEMs mirror arrays are used to perform the cross connect function.
The tilting of each mirror is controlled by applying specific electric fields to one or more of the electrodes (not shown) beneath the mirror. Undesirable variations in the gap spacing between the mirror layer and the electrode layer, symmetric or nonsymmetric, alter the electric field for the applied field, which affects the degree of electrostatic actuation and hence the degree of mirror tilting. This in turn alters the path or coherency of light signals reaching the receiving fibers, thus increasing the signal loss during beam steering.
An array of such MEMs mirrors is essentially composed of two layers: a mirror layer comprising the array of mirror elements movably coupled to a surrounding frame and an actuator layer comprising the electrodes and conductive paths needed for electrical control of the mirrors. One approach to fabricating the array is to fabricate the actuator layer and the mirror layer as successive layers on the same workpiece and then to lift up the mirror layer above the actuator layer using vertical thermal actuators or stresses in thin films. This lift-up process is described in U.S. patent application Ser. No. 09/415,178 filed by V. A. Aksyuk et al. on Nov. 8, 1999 and assigned to applicant""s assignee.
An alternative approach is to fabricate the mirror layer on one substrate, the actuator layer on a separate substrate and then to assemble the mating parts with accurate alignment and spacing. The two-part assembly process is described in U.S. Pat. No. 5,629,790 issued to Neukermans et al. on May 13, 1997 and U.S. patent application Ser. No. 09/559,216 filed by Greywall on Apr. 26, 2000, both of which are incorporated herein by reference. This two-part assembly provides a more robust structure, greater mirror packing density and permits larger mirror sizes and rotation angles as well as scalability to larger arrays.
In the two-part assembly process, the mirror layer-and the actuator layer should be bonded for mechanical sturdiness and long-term reliability. Neukermans et al. and Greywall suggest anodic bonding, solder glass bonding, and epoxy bonding. The gap spacing between the mirror layer and the actuator layer determines the electric field for the given magnitude of applied voltage (or the magnetic field for the given electrical current level). Therefore, an accurate and reliable establishment of the gap spacing during the assembly and bonding of the two parts, as well as the dimensional stability of the gap during device handling, shipping and operation are important. The accurate lateral alignment of the mating parts of the mirrors and electrodes is also desirable for reliable operation.
In accordance with the invention, a MEMs mirror device comprises a mirror layer including a frame structure and at least one mirror movably coupled to the frame and an actuator layer including at least one conductive path and at least one electrode for moving the mirror. The mirror layer and the actuator layer are provided with metallization pads and are bonded together in lateral alignment and with predetermined vertical gap spacing by solder bonds between the pads. The device has utility in optical cross connection, variable attenuation and power gain equalization.